1. Field of the Invention
The present invention relates to battery protection circuits and, more specifically, to a voltage regulator for regulating charging to a battery.
2. Description of the Prior Art
Overcharging is a universal issue that must be addressed when designing battery protection circuits. Lithium based batteries, including Lithium-Ion batteries and Lithium-Polymer batteries tend to be sensitive to excessive voltage. Without a suitable safety circuit overcharging could compromise battery reliability.
To improve reliability, several chargers and battery packs include devices that bypass the battery charging current when charging becomes excessive. Such devices detect excessive charging and, when detected, reroute the charging current through a shunt circuit. One such device is a "Zener-fuse" circuit, as shown in FIG. 1. In this circuit, a zener diode 106 begins conducting in the reverse-biased, or "avalanche," mode when the voltage from the power supply/charger 102 exceeds the normal charging voltage of the battery cells 104. Once the zener diode 106 is in avalanche mode, it acts somewhat like a short circuit relative to the power supply/charger 102. The avalanche condition causes current to increase rapidly which, in turn, causes the fuse 108 to clear, isolating the battery cells 104 from the power supply/charger 102.
The problem with this circuit is that, in reality, the zener diode 106 is not a pure short. In avalanche mode, there is actually a voltage across the zener diode 106, causing it to dissipate power. The high current required to clear the fuse also rapidly heats the zener diode 106. Thus, a "thermal race" condition exists between the zener diode 106 and the fuse 108. As both the fuse 108 and the zener diode 106 being degraded by excess heat, the zener diode 106 must experience degradation slower than the fuse 108 in order for the fuse 108 to safely clear. Therefore a zener diode 106 with a high power rating must be used to ensure that the fuse 108 clears before the zener diode 106 reaches a temperature high enough to cause it to fail. These high power zener diodes are often big, bulky and expensive.
There is therefore a need for a small, reliable, inexpensive circuit to protect against overcharging in battery cells.